Conventionally, metal castings are produced in one of three basic methods: sand mould casting, die casting, and investment casting.
In sand mould casting, a mould and any associated cores (which define cavities in the casting) are formed from bonded sand. The mould cavity is formed from one or more patterns, i.e. models of the desired casting, around which the sand is moulded and bonded (by means of organic or inorganic binders) to create a cavity of the desired size and shape in the sand. Cores (where present) are formed by so-called “core-shooting” or “ramming” in which the core is blown by pressurized gas, or manually compacted, into a mould known as a core box and bonded (in the same way as the mould) to form the desired size and shape of core. The mould is then assembled from two or more body sections of bonded sand, plus any cores, also formed from bonded sand, inserted appropriately into the mould cavity. Molten metal is then poured into the mould cavity via one or more pre-formed sprues, allowed to coo! and solidify, and the sand mould and cores are broken away to reveal the metal casting.
In die-casting, the mould is a permanent mould (i.e. it is re-used many times) and is commonly formed from metal. In investment casting, a metal die is normally used to produce a pattern of the desired shape and size of the casting formed from wax. A series of refractory (i.e. heat resistant) coatings are coated onto the wax pattern by dipping the pattern into a slurry of refractory material and allowing each coating to dry before the next coating is applied. Once a shell mould of sufficient strength has been built up by this process, the wax pattern is burnt away so as to form a mould cavity into which molten metal is poured. When the metal has solidified, the shell mould is removed to reveal the casting.
In both sand mould casting and investment casting, therefore, it is necessary to produce a pattern from which the moulds and cores may be constructed. This is a time consuming and costly process, especially if only a small number of castings needs to be made, since the proportion of the time and cost of the process devoted to the production of the patterns in such cases is clearly much greater than if a large number of castings is to be produced. This principle is most extreme in the case of rapid prototyping, in which one or a few prototype castings need to be made quickly and cheaply. Conventionally, such rapid prototype castings are generally made by firstly producing a pattern of the casting by stereo lithography, in which a photocurable resin is cured to produce the desired casting shape by means of a computer controlled laser; a sand mould and cores are then produced from the stereo lithographic pattern, and a casting is made in the sand mould. Other methods of rapid prototyping are also known, in which the pattern is made from wood, or metal, or bonded sand, for example.
In die-casting, of course, a permanent mould needs to be made, and this obviously requires a very large investment in time and money, and is only suitable for very long casting production runs. Furthermore, die-casting is only suitable for casting low melting point metals, such as aluminium, and is not suitable for high temperature castings such as those formed from iron or steel.
EP-A-0913215 relates to the production of ferrules and other feeding head and supply elements for casting moulds, insulating and exothermics, by moulding a formulation comprising aluminium silicate hollow microbeads with an alumina content below 38% by weight, an agglomerant and optional loads, in non fibrous form.
U.S. Pat. No. 5,632,326 (assigned to Foseco International Ltd) discloses moulding a composition comprising hollow alumina- and silica-containing microspheres and a binder wherein the microspheres have defined silica and alumina content.
The present invention seeks to provide a casting process, and moulds and cores, which do not require the production and use of a pattern in order to create the moulds and cores. In particular, the invention seeks to provide a casting process (and moulds and cores for use in such a process) which is especially suitable for rapid prototyping and short production run casting, although the invention is not limited to such situations, and is generally applicable to the casting of metals.
According to a first aspect, the present invention provides a process of forming a mould or care for casting metal, comprising:    (a) forming a body of material comprising, at least in part: a bonded particulate refractory material, in which the shear strength of the refractory particles is less than the shear strength of the bonds between the particles; and    (b) machining the body to form a desired mould, core, or a component thereof, in or from the body.
A second aspect of the invention provides a process of casting metal, comprising:    (a) providing a mould formed, at least in part, from one or more machined bodies of material comprising, at least in part, bonded particulate refractory material, in which the shear strength of the refractory particles is less than the shear strength of the bonds between the particles; and    (b) pouring molten metal into the mould and allowing the metal to solidify.
A third aspect of the invention provides a casting mould or core, or a component thereof, formed from a machined body comprising bonded particulate refractory material, in which the shear strength of the refractory particles is less than the shear strength of the bonds between the particles.